Generally a so-called double overhead cam shaft-type (DOHC type) valve operating system which has a pair of cam shafts for operating an intake valve and an exhaust valve, individually is conventionally adopted in a high-performance engine. In this structure, intake-side and exhaust-side cam shafts are arranged side by side on a cylinder head in a manner of rotating in synchronization with rotation of a crankshaft (an output shaft of an engine), and the intake valve and exhaust valve for each cylinder are made to reciprocate by a driving cam of each cam shaft. A pivot mechanism, such as a rocker arm, may be interposed between the driving cam and the valve.
In recent years, among the pivot mechanisms, a pivot mechanism including a so-called lost motion mechanism which does not transmit a portion of operation (motion) corresponding to the contour of the driving cam has been put into practical use. This pivot mechanism can change lift characteristics of a valve using the lost motion mechanism. In the case of incorporating such a variable mechanism, the structure of the valve operating system becomes complicated, leading to an increased number of parts. Therefore, a method of unitizing the parts of the valve operating system into a unit before installing them in a cylinder head is proposed.
For example, in the cylinder head disclosed in Patent Document 1, a cylinder head assembly is configured such that a ladder-like cam shaft holder extending over the whole upper part is provided, intake-side and exhaust-side cam shafts are rotatably supported in an upper position of the cam shaft holder while intake-side and exhaust-side rocker shafts are rotatably supported in a lower position of the cam shaft holder, and rocker arms are installed to the rocker shafts. By installing this cylinder head assembly in the cylinder head, and fastening the ladder-like cam shaft holder, the rigidity of the whole cylinder head can be increased.
Similarly, even in the variable valve operating mechanism disclosed in Patent Document 2, a ladder-like frame (intermediate member) extending over the whole upper part of a cylinder head is provided, intake-side and exhaust-side component parts of the valve operating system are assembled within the frame, and then the assembled component parts are installed in a cylinder head. In this structure, a plurality of links is combined to enable continuous changes in the lift characteristics of the intake valve. Although the structure is complicated and the number of parts is large, since the parts are assembled in advance within the frame and then the assembled parts are installed in the cylinder head, the ease of work is secured.
In regard to mass production engines, generally the sizes of the component parts of the valve operating systems may vary within a tolerance range. Similarly in a cylinder head, every cylinder may vary in size and shape. Accordingly, if the number of the component parts of the valve operating system increases like the above-mentioned conventional art, those variations may be superimposed, so that an increase in variations in lift characteristics, such as a lift amount, lift timing, and the like of valves may not be avoidable.
For this reason, in the valve gears disclosed in Patent Documents 3 and 4, a mechanism that adjusts the lift amount for each valve is provided. This mechanism measures the lift amount for each valve in a state in which the valve gear is installed in the cylinder head, and adjusts the lift amount so that the variations may decrease.